How to hightlight all ancestor menu items of a child page NOT displayed in menu

You are checking against just one parent item.

Instead, you might check against all of the parents

<?php //in functions.php
add_filter('nav_menu_css_class', 'highlight_portfolio', 12, 2);
function highlight_portfolio($classes, $item) {
    $parents = get_post_ancestors();
    if ( 0 < count($parents) ) {
        if ( in_array( $item->object_id, $parents ) {
            array_push( $classes, 'current-menu-ancestor' );
        }
    }
    return $classes;
}

Notice in your code

$parent_ID = $parent[0];

That only grabs the first parent id in the list returned. Using in_array checks to see if it matches any of the ancestors.